Have no idea on the mileage issue - Google maps should be able to answer that question. But in general, if you are talking about taking the Beltway over towards VA and around the west side of DC - don't do it due to traffic and massive construction. (The reason why I make the statement the way I do is because 495 actually goes around Washington DC on both sides - the VA side and the MD side - on the MD side, it is 95/495, so I assume that's not what you are talking about.)
And yes traffic is lessened from 10-2 (and at night 8 PM or so until about 5:30 AM), but you can't avoid construction issues.